1 - Field of the Invention
This invention relates to the synchronization of terminal stations linked to a central station via a tree-structured digital communication network. The terminal stations are at different distances from the main station. Synchronization is obtained chiefly in the central station so that data sectors or packets emitted respectively by terminal stations in communication with the central station do not overlap in time and through time multiplexing form a backward frame on receiving in the central station.
2 - Description of the Prior Art
To accomplish this synchronization the central station evaluates the propagation time through the network between the central station and each of the terminal stations, and from this deduces a delay which is transmitted to and programmable in the terminal station so that a data sector emitted by the terminal station inserts into a predetermined assigned time interval of the backward frame.
The French patent application No. 2,636,482 divulges such a synchronization method for a single-rate collective half-duplex communication network. The synchronization method relating to a terminal station according to this patent application comprises:
emitting from the central station a forward frame including a synchronization word emission order intended for the terminal station to be synchronized and having a predetermined rate,
triggering a periodical pulse counting in the central station when emitting the forward frame,
transmitting a synchronization word by the terminal station to the central station in response to the reception end of the forward frame,
halting the pulse counting in response to the detection of the synchronization word in the central station so as to derive a pulse count,
computing a delay as a function of the pulse count, and
transmitting the delay from the central station to the terminal station so that the terminal station transmits a data sector in an assigned time interval of the backward frame.
According to this method, the periodical pulses are those of a clock signal at the binary rate of the digital frames exchanged by the stations, and especially at the single operating rate of the terminal stations.
Nevertheless, the working of this network shows that the synchronization of the terminal stations in relation to a single clock signal at the rate of the terminal stations confers risks of data sectors overlapping in the backward frame. In fact, the propagation times between stations and hence the programmable delays are assessed to the nearest bit times. These assessments are all the less accurate when the stations have a high rate, and when the bit time is similar to the response times of the present day electronic components and/or variations in propagation time due to thermal drifts in the transmission network.